Photosynthesis 2.9, 8.3 Flashcards
What is the general equation of photosynthesis?
6 CO2 + 6 H2O → C6H12O6 + 6 O2
What are the ranges of wavelengths in each spectrum?
- visible light are between 400 and 700 nanometers
- longer wavelengths (infrared) = low energy
- shorter wavelengths (UV) = high energy
- plants absorb light because it is most abundant in nature
Chlorophyll
- pigment
- absorbs red and blue light
- doesn’t absorb green → reflected
- used by plants to harvest light energy
Thin layer chromatography
- Blending plant tissue and gradually adding propanone
- Putting pigment onto plastic strip
- Marking the level of pigment on the tube
- Adding running solvent.
- After 5 minutes, taking plastic strip out and measuring the distance the solvent did and the pigments did. —> Rf is the fraction of these
From this value we can recognise different pigments
Chloroplast
Outer membrane — separates from the rest (specialised compartment)
Chloroplast envelope — created by double membrane
Thylakoid — internal membrane (intense green)
Fluid filled spaces in thylakoid
Stroma — colourless fluid around thylakoid. Contains many enzymes
Grand — stacks of thylakoid
Starch grains and lipid droplets — if plant is photosynthesising quickly
Photoactivation
- photosystems = chlorophyll and other pigments together to harvest light
- Photosystem I and II
- centre → energy absorbed by chlorophyll is moved to special chlorophyll molecules
- energy from light absorbed → e- excited
- chlorophyll molecule photoactivated
- transfers e- to e- acceptor
- Photosystem II → e- acceptor = plastoquinone takes 2e-
- plastoquinone moves to the other complex
- hydrophobic → inside the membrane
- plastoquinone moves to the other complex
2 photons of light → plastoquinone
1 chlorophyll → 2e-
for 2 plastoquinones: 4e- from 2 chlorophyll molecules
Photolysis
- in thylakoid fluid
- photolysis = oxidation of water
- for chlorophyll which lost 4e-
2 H2O —> O2 + 4 H+ + 4 e-
- oxygen is a waste product → diffuses away
Where does light-dependent reaction occur?
- in thylakoid membrane (third membrane)
The electron transport chain
In photosynthesis
- photophosphorylation = producing ATP from light
- in thylakoid
- Photosystem II → electron transport chain → Photosystem I
- H+ move through ATP synthase
- plastoquinone carries electrons to the chain of electron carriers from Photosystem II
Proton gradient
In photosynthesis
- electrons pass through electron transport chain
- protons into thylakoid space → gradient created
- photolysis also adds to gradient
Chemiosmosis (photosynthesis)
- H+ down the gradient through ATP synthase
- energy released → phosphorylates ADP
- electrons are finally accepted by plastocyanin in the thylakoid fluid
Reduction of NADP
In photosynthesis
- Photosystem II passes electrons to plastocyanin
- re-excited in Photosystem I (photoactivation)
- passed to ferredoxin → 2 e- to reduce NADP → NADPH
- 12 H+ pumped into thylakoid space
- around 3 ATP molecules
- 8 photons absorbed
- supply of NADP is out
- electron moves back to chain of electron carriers
- protons pumped and energy made = cyclic photophosphorylation
- electron moves back to chain of electron carriers
Carbon fixation (step 1 of Calvin cycle)
- CO2 used to create carbohydrates
- ribulose bisphosphate (RuBP) 5 C molecule + CO2 → GP (glycerate-3-phosphate)
- catalysed by rubisco = ribulose bisphosphate carboxylase
- a lot of rubisco in stroma to maximise carbon fixation
3 RuBP + 3 CO2 —> 6 GP
Reduction of GP (glycerate-3-phosphate)
- CO2 + RuBP → GP (catalysed by rubisco)
- ratio of H:O wrong
- hydrogen added to GP (reduction)
- 6 ATP —> 6 ADP + 6P (energy) and 6 NADPH —> 6 NADP+ + 6 H+ (reducing agent)
- ATP and NADPH from light-dependent reactions
- 6 GP → 6 TP (or G3P) - triose phosphate
Regeneration of RuBP
In Calvin cycle
- RuBP must be regenerated for new cycle
- 5 TP → 3 RuBP
- only 1 TP left (after 3 cycles)
- energy 3 ATP → 3 ADP + 3 P
